High-resolution, high-speed laser scanning exposure systems for electro-photographic printers employing rotating polygonal mirror scanners and multiple beams are known in the art. In such devices, great care is normally required in the optical design of the scan lens to make the scan geometry at a final image plane insensitive to pyramid error (e.g., wobble) in the polygon mirror, and also to eliminate scan bow. Wobble may normally be prevented by bringing the multiple beams to line foci in a direction orthogonal to the scan plane at the polygon face, and then refocusing the beams at the final image plane. This process may require an anamorphic scan lens, which may be considerably more difficult to make, and hence more expensive, than a rotationally symmetric lens. This approach may also have lower performance potential due to the additional operational constraints.
Scan bow is a variation along the length of a scan between a plurality of beams in a multi-beam scanner due to the distortion characteristics of a scan lens. If not carefully controlled, the scan lens distortion together with the compound angle effect in a rotating polygon scanner may cause scan lines lying above the middle of the scan lens' field of view to be slightly concave in an upward direction, and scan lines lying below the middle of the scan lens' field of view to be slightly concave in a downward direction.
In multi-beam systems configured to write a plurality of scan lines at different vertical locations in a single horizontal swath, the scan bow may cause uppermost and lowermost scan lines to differ in shape, irrespective of placement of the swath in the field of view. When successive swaths are written, the lowermost line from one swath and the uppermost line from the next swath form adjacent lines in the final image. Differences in shape between the swaths may result in visible image defects. Such defects may be made even more visible as defects repeat periodically down a page with each swath.
Distortion of scan lens in the direction of scan is typically controlled so that a final scan coordinate is proportional to polygon scan angle θ. Since θ varies linearly in time due to continuous uniform rotation of the polygon, pixel information modulated onto write beam(s) at uniform time intervals may then be written at uniform spatial intervals. A scan lens with this distortion characteristic is commonly called an “fθ” lens. The combination of an fθ lens and a polygon scanner fails to produce straight scan lines away from the scan axis. Since in multi-beam scanning systems, all but one of the scan lines are preferred to be positioned either above or below the scan axis, such a system will exhibit scan bow.
In exemplary prior laser scanning systems, anamorphic balancing was used to provide scan bow within acceptable limits. Anamorphic balancing takes advantage of an anamorphic lens having different distortion characteristics in the two directions normal to its optic axis. The distortion experienced by a beam traversing the lens with field components in both directions may be determined by a geometric scan of the lens' two different distortion characteristics operating separately on the beam's respective field components.
Thus, beams scanned exactly along one axis or the other may encounter only the corresponding distortion characteristic. However, beams scanned along any other lines may encounter a composite distortion characteristic depending upon the relative magnitude of the beam's field components in the two directions. This approach allows compensation of the scan bow due to the distortion along the scan, axis in a narrow region near the scan axis by a large distortion in the orthogonal direction of the opposite sign of the scan bow.
Anamorphic balancing places additional demands upon the scan lens and restricts the degrees of freedom that may be used to satisfy other demands, such as increases in the format width and the number of resolvable spots desired of the lens.
An alternative approach to scan bow control uses a rotationally symmetric scan lens with an fsinθ distortion characteristic. A scan lens with this distortion exactly compensates for the scan bow characteristic of the rotating polygon mirror, resulting in zero net scan bow for scan lines placed anywhere within the field. However, in such a system, the final scan coordinate may not be proportional to “θ”, and information may have to be modulated onto the beam(s) at non-uniform time intervals in order to be written at uniform spatial intervals.
FIG. 1A illustrates a plan view of a prior art arrangement 100 to overcome the inherent wobble correction defect found in a system of the fsinθ type as described above. Wobble correction defect may be overcome by using arrangement 100 in order to permit a beam of light to double bounce off of a scanning device 102 (e.g., rotating polygon mirror).
Continuing to refer to FIG. 1A, a roof reflector 104 oriented with a roof intersection line in the plane of scan may be used to re-direct a scanned beam to the same polygon face of the scanning device 102 for a second reflection. The double bouncing of a beam of light has the effect of removing any change of angle in the cross-scan direction that may have been imparted to the beam due to polygon pyramid error on the first bounce from the scanning device 102.
FIG. 1B shows an elevation view of roof reflector 104 of arrangement 100 shown in FIG. 1A. Disadvantages exist with the arrangement of FIG. 1A providing the self-correction action and include utilization of a larger polygon when compared to a polygon used in a single bounce system, and particularly so if the scan angle is large. For example, beam diameter at the input to an fsinθ scan lens required for a scanner desired to cover a 500 mm wide format with a ±30° scan to produce a ˜45 micron spot diameter (FW @ 1/e2) is about 15 mm. For FIG. 1A, the angle between an incoming beam to the polygon 102 and an outgoing beam to the roof reflector 104 having the minimum polygon facet length was found to be around 50°. An angle of 45° between an incoming beam to the polygon 102 may require the roof reflector 104 to be placed farther away from the polygon 102 to clear the lowermost scanned beam, thus requiring both a larger roof and a larger facet because of the angular spread of the scanned beams. A scan angle of 55° allows a slightly smaller roof to be placed closer to the polygon 102, but again requires a larger facet because of the more oblique second bounce). The resulting deflection system is shown in FIG. 2.
FIG. 2 shows a double bounce deflection system 200 using a conventional roof reflector 204. Polygon 202 may have to be designed to have facets that are 126 mm in width. For example, for a 12-sided polygon, the facet width would correspond to a diameter of 470 mm (>18″). Such a large polygon would be relatively expensive to manufacture, and may be difficult to spin at speeds as high as 16K rpm without serious distortion of the optical figure of the facets.
FIG. 3 shows a conventional single-bounce deflection system 300, using a polygon 302, for the same beam size and angular scan range illustrated in FIG. 2. The deflection system 300, however, may need a polygon having only six facets, individual facets having a width of only 50 mm, the width corresponding to a polygon diameter of 86.6 mm. This leads to increased cost and complexity, as well as limitations with respect to optical performance (i.e., such a system may not cover as wide a field at a given (high) resolution).